If you're playing the Xbox 360 version in Skulls Online, and running into errors joining games, it may be a NAT configuration issue.
Your network's router NAT setting may block joins from being joined - unfortunately the game can't detect that this is the problem, so the error isn't very informative.
You can follow these instructions here to see if that's the problem: http://support.xbox.com/en-US/xbox-live/connecting/nat-type-strict